This invention relates generally to spacecraft communication systems and, in particular, to spacecraft communication systems that have at least one spacecraft that receives uplink signals from a number of ground station transmitters located within a particular beam, and that frequency shifts and retransmits the received uplink signals to receivers located within the same particular beam.
The use of a geosynchronous orbit satellite to broadcast television signals to terrestrial receivers is well known in the art. By example, reference can be had to the following two publications: xe2x80x9cFlight Hardware Test Results Obtained on High Power Equipment and on the Repeater Subsystem of 12 GHz DBSxe2x80x9d, W. Liebisch et al., 86-0646 AIAA, pp. 266-274, 1986; and xe2x80x9cThe Thermal Control System of the German Direct Transmitting Communication Satellite TV-SATxe2x80x9d, Kreeb et al., AIAA 8th Communications Satellite Systems Conference, Apr. 20-24, 1980.
A number of problems are presented in the design of a high performance satellite communications system that provides, for example, television service to terrestrial receivers spread over a large geographical area. In such a system a number of different ground stations, each associated with a different locale and demographic market, may each transmit an uplink signal that is intended to be received by a spacecraft, such as a geosynchronous orbit satellite, and then transmitted, through one or more transponder channels, from the spacecraft to television receivers within the locale served by the ground station. For example, one ground station may serve the New York City area, another may serve the St. Louis area, while another serves the Salt Lake City area. Each ground station can provide one or more television channels, and is considered to be located within a particular spacecraft beam. More than one ground station can be serviced by a particular beam.
As can be appreciated, in such a system the size and hence downlink power requirements of each service area may differ significantly. That is, for a predetermined amount of RF power at the ground, more spacecraft transmitter power is required for a large beam than for a small beam. Furthermore, and in order to maximize the total number of ground stations that can be serviced, the spacecraft will require a significant number of uplink receivers, as well as a significant number of downlink power amplifiers, typically implemented as Travelling Wave Tube Amplifiers (TWTAs). In addition, it is important to provide some capability to control the transmission power so as to compensate for localized signal impairments, typically rain attenuation, that may be experienced at any given time in one locale but not in others.
It is known to provide gain and RF power control of transponder channels on one downlink beam with one ground station, but not with gain and RF power control of different transponder channels with multiple ground stations in a downlink beam.
In accordance with the prior art, and referring to FIG. 1A, a spacecraft communication system may have a spacecraft 1 that uses separate TWTAs 2 that each receive a separate signal from ground stations (GSs) located within the same or different beams. By example, a first beam (beam #1) may include four ground stations (GS1-GS4) while a second beam (beam #2) may include six different ground stations (GS1-GS6). Each ground station signal is passed through a separate spacecraft transponder channel, which includes a channel amplifier circuit, shown generally as an amplifier 4, and a TWTA 2. Each channel amplifier circuit 4 may be separately gain and/or RF power controlled by the associated ground station. The outputs of the TWTAs 2 for each beam are combined in an output multiplexer (OMUX) 3 prior to transmission on the downlink to the terrestrial receivers in each regional or spot beam.
It can be realized that this conventional approach can be wasteful of power and TWTAs, as each transponder channel will typically have differing RF power requirements. If it were desired to use only one type of TWTA (e.g., a 60 W TWTA) or only two types (e.g., 60 W and 120 W), then a transponder channel that requires only 10 W of RF power will use its TWTA much less efficiently than another transponder channel that requires 50 W of RF power.
Further in accordance with the prior art a single size spot beam may be provided that is contiguous across the continental United States (CONUS). Alternatively, and as is exemplified by U.S. Pat. No. 4,819,227, xe2x80x9cSatellite Communications System Employing Frequency Reusexe2x80x9d to H. A. Rosen, a two-way satellite communication system can use spot beams in contiguous zones. In general, the prior art requires either more satellites or larger spot beam spacing, using a single size of spot beams, to obtain a required performance. The prior art may as well use more antennas interlaced over the CONUS area, with larger feed spacings and thus require more area on the satellite.
It is also known from the prior art to provide as many receivers as the total number of transponder channels, or as many as the number of feeds/beams, and to have each receiver translate its associated transponder channel or feed/beam to the appropriate downlink channel frequencies. Referring to FIG. 1C, the prior art teaches a system that uses either a single receiver 7 for one transponder or a single receiver 7 for one feed or one beam. As was also the case for FIG. 1A, each GS signal may originate from a separate geographical area (e.g., from ground stations located in different urban areas).
As can be appreciated, and as was also the case for FIG. 1A, the prior art approaches are not efficient with regard to spacecraft power consumption, weight, and/or payload utilization.
It is a first object and advantage of this invention to provide an improved satellite communications system wherein a plurality of satellite transponder channels are combined and amplified by a single high power amplifier, such as a TWTA or multiple paralleled TWTAs.
It is another object and advantage of this invention to provide an improved satellite communications system wherein each ground station in a spot or regional beam has individual RF power control for its associated transponder channel or channels, enabling an adjustment of downlink power due to rain attenuation within the spot or regional beam.
Certain of the foregoing and other problems are overcome and the objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.
In accordance with this invention there is provided a satellite communication RF power control system to deliver digital data, such as digital television data, from multiple ground stations to a single spot or regional beam. Each ground station in the spot or regional beam has separate and individual control of the RF power for their transponder channel or channels. The RF power control for each individual ground station enables adjustment of downlink power due to rain attenuation within the spot or regional beam. That is, power adjustments are performed in a particular transponder channel prior to combination with other transponder channels and amplification by a single, common TWTA or multiple paralleled TWTAs.
The use of this invention enables multiple ground stations with assigned transponder channels to have RF power and gain control in an assigned spot or regional beam, with minimal effects to neighboring ground station signal(s). In the inventive satellite communications system there is a sharing of the amount of gain and power control between the ground station and the spacecraft. For a given spot or regional beam there is determined a number of ground stations that can share a common TWTA, without exceeding the capability of the TWTA under worse case conditions, and then an assignment is made of separate channel amplifiers for each ground station. A summation of the channel amplifier outputs is applied to a common linearizer that drives a common TWTA or multiple paralleled TWTAs. This technique enables a single type and size of TWTA to be used on a given spacecraft, thereby lowering cost and complexity, as well as power consumption.
In this embodiment an analysis of multiple transponder channels in a nonlinear system is performed to determine the number of transponders that can be used for each of the multiple ground stations, each having their own spacecraft channel amplifiers, to drive a linearized TWTA or multiple paralleled TWTAs into one beam. This technique thus further eliminates output multiplexer losses which can directly impact the spacecraft""s power and thermal efficiency.
An example of this invention is a method of distributing digital data, such as digital television data, using multiple spot beams with different shapes and sizes to fully cover and serve designated market areas (DMAs) using multiple spacecraft in a geosynchronous orbit. Each spot beam can contain multiple ground stations. Each ground station can be assigned multiple transponder channels and has the ability to adjust transponder channel power or gain. By assigning a channel amplifier for each ground station, multiple ground stations can share a TWTA or multiple paralleled TWTAs.
This invention thus provides a satellite communication system that includes at least one but preferably a plurality of spacecraft in geosynchronous orbit, where each spacecraft provides a plurality of beams on the surface of the earth, and a plurality of ground stations individual ones of which are located in one of the beams for transmitting uplink signals to one of the spacecraft. Each spacecraft has a plurality of receivers for receiving a plurality of the uplinked signals from ground stations, a frequency translator for translating the received uplink signals to a transmission frequency of a plurality of downlink signals, and a plurality of transmitters for transmitting the plurality of downlink signals within the same beams as the corresponding uplink signals. Each transmitter includes a combiner for combining together a plurality of frequency translated signals and a power amplifier, such as a TWTA, for amplifying the combined plurality of frequency translated signals. Each of the plurality of receivers is gain controlled by a corresponding one of the ground stations for providing immunity to rain attenuation. Preferably the beams have different sizes and shapes, and are non-contiguous over a least a portion of the earth""s surface. It is also preferred that the uplink beams to a first one of the satellites have a first polarization and downlink beams have a second, opposite polarization, and that uplink beams to a second one of the satellites have the second polarization and downlink beams have the first polarization. The uplink and downlink signals may be digital television signals, and in this case individual ones of the spot beams are placed over a predetermined one of a designated television market area.